1. Field of the Invention
The present invention relates to a method of quenching a workpiece with a hole, such as a crankshaft for use in internal combustion engines, to harden the workpiece, an assistive tool for use in quenching such a workpiece, and a crankshaft.
2. Description of the Related Art
Internal combustion engines for use on motor vehicles have a crankshaft that is connected to pistons by connecting rods. The crankshaft comprises a plurality of pins, a plurality of journals, and a plurality of counterbalances. The journals are rototably supported by respective shanks, and the connecting rods are rotatably coupled to the pins, respectively. The journals and the pins have oil holes defined therein for the passage of lubricating oil. The oil holes provide a lubricating oil passage for supplying part of the lubricating oil from bearings to the pins and the journals to lubricate bearing shells in the connecting rods.
The pins and the journals are case-hardened by high-frequency induction hardening in order to increase the surface hardness thereof.
One problem of the crankshaft is that because of the lubricating oil passage, the pins which are positioned out of alignment with the journals tend to have their torsional fatigue strength reduced.
The problem may be solved by effecting high-frequency induction hardening on the pins and the journals and regions around the oil holes under different conditions to produce a deeper hardened layer in the regions around the oil holes.
There is also known a technical approach to increase the fatigue strength of a crankshaft near openings of a lubricating oil passage defined therein by effecting high-frequency induction hardening on the surface of the crankshaft which includes the openings of the lubricating oil passage and then effecting laser beam quenching or shot peening on surface regions in the openings beneath the hardened layer that has been produced by the high-frequency induction hardening (see Japanese laid-open patent publication No. 9-14252).
The former solution, i.e., subjecting the pins and the journals and the regions around the oil holes to high-frequency induction hardening under different conditions, is disadvantageous in that the production efficiency is low and the heating coil used for high-frequency induction hardening has its service life greatly reduced, tending to cause increased strains in the crankshaft.
The latter solution, i.e., the approach disclosed in Japanese laid-open patent publication No. 9-14252, is also problematic in that since a laser beam quenching device is required in addition to the high-frequency induction hardening device, it results in added investments in the production facility, which tend to increase the manufacturing cost of the crankshaft, and it is also apt to greatly lower the production efficiency.
It is a general object of the present invention to provide a method of quenching a workpiece with a hole to produce a quenched hardened layer on the outer surface of a workpiece and simultaneously produce a quenched hardened layer in the hole including an opening thereof.
A major object of the present invention is to provide an assistive tool which is capable of easily producing a quenched hardened layer in a hole defined in a workpiece.
Another object of the present invention is to provide a crankshaft which has a desired hardened layer produced by high-frequency induction hardening entirely or partly in an oil hole defined in the crankshaft, depending on the durability required of the crankshaft.
The above and other objects, features, and advantages of the present invention will become more apparent from the following description when taken in conjunction with the accompanying drawings in which preferred embodiments of the present invention are shown by way of illustrative example.